422 NORTH AMERICAN INDEX FOSSILS. 



come wholly atrophied as in the Hemiptera. The preceding three 

 veins constitute the costo-radial group, arising from a distinct 

 trunk (Fig. 1724). The cubitus and anal veins constitute the 

 cubito-anal group. The media (M), also called the externo- 

 mcd'iaii (Fig. 1737), is a member of the costo-radial group in the 

 primitive or retarded forms, but migrates to the cubito-anal group, 

 or arises from a transverse basal connecting trachea in all the more 

 specialized groups. This connecting trachea, as well as the bases 

 of the wing trachea are within the thorax of the adult insect, and 

 do not appear in the veining of fossil wings. 



The media is usually four-branched in the generalized members 

 of widely separated orders, though in some primitive forms it is 

 only three-branched. The branches are numbered from before 

 backwards. In certain specialized forms, the number of branches 

 is much greater, there being repeated furcations, while in others 

 a reduction takes place which may be caused by the disappearance 

 of the main stem of the media, as in many Lepidoptera. 



The cubitus or fifth principal vein (also called internomedian, 

 Fig. 172)7) separates into two branches in the primitive type. This 

 number may, however, become greatly increased as in the cock- 

 roach wing (Fig. 1737) where there are twelve. 



The posterior group of veins comprises the anal veins {A). 

 They lie between the cubitus and the posterior margin of the wing. 

 In primitive or immature types, there are three, which may arise 

 independently from the cubito-anal stem. In specialized forms 

 they are increased by furcation, or decreased by coalescence, or 

 by atrophy of one or more of the veins, accompanied by a decrease 

 of the anal area. 



Besides disappearance of veins through atrophy, there often 

 results a reduction of the number of veins as a whole by coales- 

 cence, the point of furcation migrating towards the margin of the 

 wing, until it disappears, or the veins unite at their tips, this coales- 

 cence then extending backwards. Veins may also coalesce for 

 part of their extent, and be free at both extremities. 



Cross-veins often connect the longitudinal ones. They arise 

 as a rule secondarily, and are not preceded by tracheae. A few 

 of these cross-veins appear sufficiently constant to receive distinct 

 names (Fig. 1725). These are: (i) humeral cross-vein, extend- 



